Harvest weed seed control is an alternative non-chemical approach to weed management that targets escaped weed seeds at the time of crop harvest. Relatively little is known on how these methods will work on species in the US. Two of the most prominent weeds in soybean production in the midsouthern US are Palmer amaranth and barnyardgrass. Typically, when crop harvesting occurs the weed seed has already either shattered or is taken into the combine and may be redistributed in the soil seedbank. This causes further weed seed spread and may contribute to the addition of resistant seeds in the seedbank. There is little research on how much seed is retained on different weed species at or beyond harvest time. Thus, the objective of this study was to determine the percentage of total Palmer amaranth and barnyardgrass seed production that was retained on the plant during delayed soybean harvest. Retained seed over time was similar between 2015 and 2016, but was significantly different between years for only Palmer amaranth. Seed retention did not differ between years for either weed species. Palmer amaranth and barnyardgrass retained 98 and 41% of their seed at soybean maturity and 95 and 32% of their seed one month after soybean maturity, respectively. Thus, this research indicates that if there are escaped Palmer amaranth plants and soybean is harvested in a timely manner, most seed will enter the combine and offer potential for capture or destruction of these seeds using harvest weed seed control tactics. While there would be some benefit to using HWSC for barnyardgrass, the utility of this practice on mitigating herbicide resistance would be less pronounced than that of Palmer amaranth because of the reduced seed retention or early seed shatter.
Narrow-windrow burning has been a successful form of harvest weed seed control in Australian cropping systems, but little is known about the efficacy of narrow-windrow burning on weed seeds infesting U.S. cropping systems. An experiment was conducted using a high-fire kiln that exposed various grass and broadleaf weed seeds to temperatures of 200, 300, 400, 500, and 600 C for 20, 40, 60, and 80 s to determine the temperature and time needed to kill weed seeds. Weeds evaluated included Italian ryegrass, barnyardgrass, johnsongrass, sicklepod, Palmer amaranth, prickly sida, velvetleaf, pitted morningglory, and hemp sesbania. Two field experiments were also conducted over consecutive growing seasons, with the first experiment aimed at determining the amount of heat produced during burning of narrow windrows of soybean harvest residues (chaff and straw) and the effect of this heat on weed seed mortality. The second field experiment aimed to determine the effect of wind speed on the duration and intensity of burning narrow windrows of soybean harvest residues. Following exposure to the highest temperature and longest duration in the kiln, only sicklepod showed any survival (<1% average); however, in most cases, the seeds were completely destroyed (ash). A heat index of only 22,600 was needed to kill all seeds of Palmer amaranth, barnyardgrass, and Italian ryegrass. In the field, all seeds of the evaluated weed species were completely destroyed by narrow-windrow burning of 1.08 to 1.95 kg m−2 of soybean residues. The burn duration of the soybean harvest residues declined as wind speed increased. Findings from the kiln and field experiments show that complete kill is likely for weed seeds concentrated into narrow windrows of burned soybean residues. Given the low cost of implementation of narrow-windrow burning and the seed kill efficacy on various weed species, this strategy may be an attractive option for destroying weed seed.
Harvest weed seed control (HWSC) tactics are being investigated for herbicide resistance management by reducing the number of weed seeds entering the soil seedbank. Weed seed retention and the location where weed seeds exit the combine (chaff, straw, or grain) are factors influencing potential HWSC success. An experiment was conducted in 2014 and 2015 in Keiser, AR, to determine where the seeds of common cocklebur (Xanthium strumarium L.) and Palmer amaranth (Amaranthus palmeri S. Wats.) exited the combine during soybean [Glycine max (L.) Merr.] harvest. Plots within soybean production fields containing infestations of common cocklebur and Palmer amaranth were harvested with a combine and the grain, chaff (top sieve), and straw (rotor) fractions were collected. The number of seeds in each fraction was determined for both species. The grain fraction contained 6 and 13% of the total Palmer amaranth and common cocklebur seed entering the combine, with 75 and 85% of the common cocklebur and Palmer amaranth seed, respectively, in the chaff fraction. As the chaff fraction is the target for many HWSC systems (e.g., integrated Harrington Seed Destructor, chaff carts, chaff lining, and chaff tramlining), these proportions indicate the potential success of these systems. The HWSC systems that target all harvest residues (e.g. narrow windrow burning and bale direct) would potentially control high proportions of common cocklebur and Palmer amaranth seed (94 and 87% respectively) during harvest. Preventing this seed from entering the seedbank would improve weed management programs and lessen the likelihood of selecting for herbicide resistance.
A three-year field study was conducted in Keiser, Arkansas to investigate the response of the naturally occurring weed flora, dominated by Palmer amaranth, under various combinations of 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide-based programs and crop rotation sequences. The first year, corn plots were established with three corn HPPD-based herbicide programs designed to represent a range of efficacies and selection pressures for resistance. The following two years, corn as monoculture or with soybean and/or cotton crops were included in rotation sequence for selected herbicide programs. Weed emergence, weed biomass, and soil seedbank were assessed through the entire experimental period. The results showed that crop rotation, especially a rotation sequence with corn followed by (fb) soybean fb cotton, and lowest risk herbicide program involving seven sites of action over the course of the entire crop rotation, was effective in reducing the emergence of naturally occurring weeds, including Palmer amaranth, prickly sida, morningglory species, and grass weeds (broadleaf signalgrass, large crabgrass, barnyardgrass, and johnsongrass) by 88.3, 57.5, 28.7, and 76.3% respectively Treatments without crop rotation (corn as monoculture for three consecutive years) and poor herbicide programs, with one site of action, increased weed emergence, notably Palmer amaranth and prickly sida by 73.5 and 74.1 respectively. The soil seedbank showed a similar trend to weed emergence. This study highlights the fact that reducing the weed seedbank cannot rely on one management practice but requires the use of a multi-tactic approach with various control methods. HPPD-inhibiting herbicide programs seem to be effective on Palmer amaranth when coupled with crop rotation and should be used with other best management practices.
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